1. Field of the Invention
The present invention relates generally to electronic components, and more particularly to a method of attaching an electronic component to a flexible substrate.
2. Disclosure Information
Ball grid array (BGA) chips are surface mount electronic components having ball-shaped "bump" terminations on a bottom surface thereof. These terminations are typically formed by a process known as "ball bumping", which is well known in the art of electronics manufacturing. The bumps may be made of a metal (e.g., gold, aluminum) having a melting point well above any processing temperatures to which the BGA will be exposed, or they may be made of solder (e.g., eutectic tin/lead). In the former case, the bumps may optionally be provided with an outer coating of solder.
BGA chips are typically attached to circuit traces on a substrate by one of three approaches, as illustrated in FIG. 1. The first approach involves providing a BGA 70 having eutectic solder bump terminations 73 with flux 74 applied atop the corresponding circuit trace mounting pads 76, the second approach involves providing a BGA 71 having gold or other high-melting-point metal bumps 75 with solder paste 78 (i.e., solder plus flux) applied atop the mounting pads 76, and the third approach involves providing a BGA 72 having gold/other metal bumps 75 coated with solder 73 with flux 74 applied atop the mounting pads 76. In each approach, the EGA 70/71/72 is placed on the substrate 77 with the bumps 73/75 resting atop their respective circuit trace mounting pads 76 (amid flux 74 or solder paste 78), and then the entire assembly is shuttled through a reflow oven in order to melt, flow, and solidify the solder so as to mechanically and electrically connect each termination with its respective circuit trace.
BGAs are usually attached to rigid, flat, planar substrates made of FR-4 glass/epoxy composite or similar materials, although in some applications BGAs may instead be attached to flexible substrates made of polyimide, polyester, or other like materials. However, flexible substrates are much more sensitive than rigid FR-4 substrates to the elevated temperatures to which the assembly is exposed during reflow. Such elevated temperatures may cause thermal degradation and other problems in the flexible substrate material.
Various methods have been proposed for protecting the flexible substrate from excessive thermal exposure during reflow processing. However, it would be desirable to provide a method for attaching a EGA to a flexible substrate without having to subject the substrate/BGA assembly to the reflow process at all.